This invention relates generally to radiotelephones and, in particular, to radiotelephones or mobile stations capable of operation with a digital wireless telecommunications network.
FIGS. 1A, 1B and 1C show the frame and time slot formats for an exemplary prior art digital Time Division Multiple Access (TDMA) cellular air interface known in the art as IS-136 (see, for example, IS-136.1, Rev. A, Mar. 21, 1996 and IS-136.2, Rev. A, Feb. 12, 1996, as well as later revisions).
FIG. 1A shows that a 40 millisecond frame consists of six time slots. Slots 1-3 and 4-6 each comprise one TDMA Block. In the forward direction from a base station to a mobile station the frames are continuously transmitted. A given mobile station is assigned to receive in one time slot per frame for a half data rate case, and is assigned to receive in two time slots for a full data rate case. FIG. 1B illustrates the format of one slot in the reverse direction from the mobile station to the base station, while FIG. 1C illustrates the format of one slot in the forward direction from the base station to the mobile station. The base station forms a part of a Base Station/Mobile Switching Center/Interworking function (BMI). Of most interest herein is the forward direction from the base station to the mobile station.
In a conventional implementation of IS-136 the modulation scheme uses a xcfx80/4-shifted DQPSK constellation as shown in FIG. 1D, wherein a Gray code is used in the mapping; i.e., two di-bit symbols corresponding to adjacent signal phases differ only in a single bit. In this modulation technique the information is differentially encoded and symbols are transmitted as changes in phase rather than absolute phases.
In an enhanced version of IS-136 (TIA IS-136, Rev. C) a total of four coherently modulated (8PSK) pilot symbol sequences are defined per time slot. Reference in this regard can be had to FIG. 1E, which illustrates a currently proposed slot format. Each pilot symbol sequence consists of three symbols. In this enhanced TDMA system there is a capability for double rate data as well as triple rate data in various combinations, which are useful in transmitting packet data.
More particularly, in a Global Packet Radio System (GPRS) specified for a version of IS-136 of most interest to this invention, and still referring to FIG. 1E, a Header section is 32 symbols in length and a Data portion is 130 symbols in length (a two symbol trailing Ramp portion is not illustrated). The xcfx80/4-shifted DQPSK modulation is always used in the Header portion, and either xcfx80/4-shifted DQPSK modulation or coherent 8-PSK modulation is used in the Data portion. Before the detection of the received slot can be performed, the receiver must determine which type of modulation is in use in the Data portion of the time slot.
The Header includes 14 synchronization (SYNC) symbols, six Coded Data Field Type (CDFT) symbols, and 12 Packet Channel Feedback (PCF) symbols. The CDFT field contains the following eight bits:
(b2,b1,b0,SFP4,SFP3,SFP2,SFP1,SPF0).
The bit b2 indicates whether incremental redundancy or fixed coding is used in the time slot. The bits b1 and b0 are of most interest to this invention, as they provide an indication the type of modulation that is used in the following Data portion of the time slot. The bits SFP0-SFP4 indicate the superframe phase (0-31). This eight bit word is encoded to 12-bits (six symbols) using a (12,8) code.
When coherent 8-PSK modulation is used, the data portion of the time slot has the following structure:
data(17),pilot(3),data(33),pilot(3),data(33), pilot(3),data(33),pilot(3).
There are thus four data fields and four pilot fields, with the pilot symbols being known symbols used for channel estimation.
When xcfx80/4-shifted DQPSK modulation is used instead of 8-PSK modulation, the Data portion of the time slot format includes 128 data symbols. No pilot symbols are used, and in their place can be found random symbols.
A problem occurs in the packet data mode, in particular the GPRS mode specified for IS-136, as the receiving mobile station does not know whether xcfx80/4-shifted DQPSK or coherent 8-PSK modulation is used in the Data portion of a received time slot. As was stated above, the correct modulation is intended to be indicated by detecting the two bits b1 and b0 of the six symbol CDFT field found in the Header portion. However, if for any reason the detection of these two bits fails (e.g., due to channel fading, the presence of impulse noise, etc.), then the wrong modulation type can be selected, resulting in the Data portion of the time slot being lost.
It can be appreciated that it would be beneficial to provide a more robust and reliable technique to determine the modulation type of a received time slot in packet data radio telecommunications system, thereby reducing the occurrence of lost time slots and the resulting requirement to retransmit the lost data.
It is thus a first object and advantage of this invention to provide a technique to overcome the aforementioned problems by avoiding the necessity to determine a type of modulation used in a received time slot from the bits of the CDFT field in the time slot header.
It is a further object and advantage of this invention to provide a technique for detecting a type of time slot modulation that is in use by employing pilot symbols found in the data portion of the 8-PSK time slot structure.
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
A method of this invention is disclosed for operating a wireless mobile or user station to receive a time slot sent through a radio channel from a transmitter, the time slot having a first portion and a second portion. The first portion contains information for specifying whether the second portion is modulated using a first modulation type or a second modulation type. When modulated using said second modulation type, said second portion contains a predetermined symbol sequence at a predetermined location. The method has steps of (a) receiving the time slot; (b) operating a demodulator for use with the second type of modulation and determining an amount of detected signal energy, assuming that the predetermined symbol sequence is located at the predetermined symbol sequence location; (c) comparing the power of the detection error to a threshold value to derive a modulation type indication; and (d) based at least in part on the modulation type indication, specifying that the second portion of the received time slot is modulated with the first modulation type or that the second portion is modulated with the second modulation type. The step of operating a demodulator operates a coherent demodulator, preferably a demodulator that operates as an 8-PSK demodulator. The first portion of the time slot containing the information is a CDFT field.
When it is specified that the second portion of the received time slot is modulated with the second modulation type, the method further includes a step of operating a coherent detector to detect the second portion of the time slot.
When it is specified that the second portion of the received time slot is modulated with the second modulation type, the method may contain further steps of operating a second detector to detect the first portion of the time slot containing the information and, using the detected information, verifying that the second portion of the received time slot is modulated with the second modulation type.
The specifying step may further include steps of operating the second detector to detect the first portion of the time slot, which contains the information, to derive a further modulation type indication, combining the modulation type indication and the further modulation type indication and, based on the combined modulation type indications, specifying that said second portion of the received time slot is modulated with the first modulation type or that said second portion is modulated with the second modulation type.
When it is specified that the second portion of the received time slot is modulated with the second modulation type, the method may further include steps of operating the second detector to detect the first portion of the time slot containing the information and, using the detected information, verifying that the second portion of the received time slot is modulated with the second modulation type.
Also disclosed is an algorithm for operating a wireless mobile station to receive a time slot sent through the radio channel from the transmitter. In a GPRS embodiment the time slot has a Header portion and a Data portion, wherein the Header portion contains information for specifying whether the Data portion is modulated using 8-PSK modulation or xcfx80/4-shifted DQPSK modulation. When modulated using 8-PSK modulation, the Data portion includes a plurality of Pilot symbol sequences at predetermined locations. The method executed in accordance with the presently preferred algorithm performs steps of: (a) estimating the quality of the radio channel using symbol sequences found at predetermined locations corresponding to the Pilot symbol sequence locations in the Data portion of the time slot; (b) determining a magnitude of a detection error between assumed transmitted Pilot symbol sequences and the detected symbols found at the predetermined locations; (c) calculating a mean of the detection error powers; (d) comparing the calculated mean of the detection error powers with a threshold value; and (e) selecting the modulation type of the Data portion as being one of 8-PSK or xcfx80/4-shifted DQPSK based on the result of the step of comparing.
In these embodiments an additional step of varying a magnitude of the threshold value as a function of at least one characteristic of the radio channel can be performed, such as by varying the magnitude of the threshold value as a function of the power of the radio channel noise, as determined from a predetermined symbol sequence (SYNC) that forms a part of the Header portion.